image credit: Ed Lee
A typical TV antenna such as this one (hi-res version) is a resonant system--the transverse arms are half a wavelength long at the frequencies of the traditional broadcast TV channels. The smaller sets of transverse arms are for higher frequency (UHF) channels. For more on resonance, see Standing Wave Resonance. To read about a new kind of antenna, visit Fractal Antenna Design.
(This feature was updated on July 10, 2013.)
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Coupled Resonant Pendulums
For a nice example of resonance, try the Exploratorium activity at Coupled Resonant Pendulums. You'll use everyday materials to show the exchange of energy between two pendulums. The site provides a thorough description of how the two pendulums interact, contrasts their motion, and presents an easy way to find the rate of energy exchange. For a nice simulation, see Two Coupled Pendula.